US Secretary of State John Kerry said Monday it was "urgent" to set a date for the so-called Geneva 2 meeting on the future of Syria but that peace was impossible until President Bashar al-Assad was replaced.

After talks with UN special representative Lakhdar Brahimi in London, Kerry said "we believe that it is urgent to set a date to convene the conference and work toward a new Syria."

"For our part the United States are deeply committed to try to set a date very soon," Kerry said.

"President Assad has lost the legitimacy necessary to be able to be a cohesive force, that could bring people together" and therefore "there has to be a transition government, there has to be a new governing entity in Syria."

"This will require all the parties to come together in good faith," Kerry said.

He said that when Brahimi travels to the region after this week's Eid al-Adha holiday, he will "meet with all of the relevant countries, as well as the relevant parties."

"And he will be working on the question of the process for a Geneva 2 conference," Kerry said.

Brahimi said: "I am going to the region immediately after Eid to see as many people as I can to discuss with them, to hear from them what are their preoccupations, what are their ideas and how they can contribute to this Geneva conference."

Russia on Monday urged the United States to do everything in its power to persuade the Syrian opposition to take part in peace talks after a key group said it would not attend the proposed conference in Geneva.

The first Geneva conference took place in the Swiss city in June 2012.